A waveguide is a spatially inhomogeneous structure for guiding light, i.e. for restricting the spatial region in which light can propagate. This can be used e.g. for transmitting light over long distances (e.g. in telecommunication systems), for guiding light on integrated optical chips (silicon photonics), for maintaining high optical intensities of appreciable lengths (e.g. in waveguide lasers and frequency doublers), for stripping off higher-order transverse modes, for interaction of the guided light with material in the evanescent field (e.g. in certain waveguide sensors), or for splitting and combining beams. Usually, a waveguide contains a region of increased refractive index, compared with a surrounding medium. However, guidance is also possible by the use of reflections e.g. at metallic interfaces.
There are many different techniques for fabricating waveguides. Some examples are lithographic techniques, used with semiconductor, crystal and glass materials, in combination, e.g. with ion exchange or thermal interfusion, fiber fabrication by drawing from a pre-form, drawing fibers into waveguides of further reduced dimensions, resulting in nano-wires, writing of waveguides in transparent media with focused and pulsed laser beams, exploiting laser-induced break-down and epitaxial and polishing methods for fabrication of planar waveguides. The trade-offs between different fabrication techniques can be complicated. They can involve aspects like cost, flexibility and reproducibility of manufacturing, achieved propagation losses, possible side effects on the material (e.g. via heating or undiffused materials), optimum mode size and symmetry for coupling to other waveguides.
Planar waveguides are waveguides with a planar geometry. They are often fabricated in the form of a thin transparent film or layer with increased refractive index on some substrate, or possibly embedded between two substrate layers.
Thus, there is still a need for further material systems which provide reliable methods to produce optical waveguides with low absorption loss.
The object is solved by a method, an optical waveguide structure, and a sensor arrangement according to claims 1, 12 and 18 respectively.
Further embodiments are defined in the dependent claims.
Further details of the invention will become apparent from the consideration of the drawings and ensuing description.